1. Field of the Invention
The present invention relates to a coil rotation preventing structure installed on a side portion of a lens holder for an optical pick-up actuator.
2. Description of the Related Art
Generally, an optical pick-up actuator functions to constantly maintain a relative distance between an object lens and an optical medium (i.e., a disk) by moving structural elements (a lens holder, a bobbin, a coil, and the like) including the object lens. In addition, the optical pick-up actuator runs along a track formed on the optical medium to write/read the information on/from the optical medium.
FIG. 1 shows a prior optical pick-up actuator.
Referring to FIG. 1, a prior optical pick-up actuator 200 includes a lens holder 202, on a center of which an object lens 201 is movably mounted and magnetic circuit assemblies disposed around the lens holder 202.
The magnetic circuit assembly includes tracking and focusing coils 205 and 206, a tilt coil 217, a yoke 203 and multi-polar magnet assemblies 204 disposed facing opposite surfaces of the lens holder 202.
The focusing coils 205 are disposed on left and right sides of the opposite surfaces of the lens holder 202, opposing a vertical border of the magnet polarity. The tracking coils 206 are disposed between the focusing coils 205 on the both surfaces of the lens holder 202, opposing a horizontal border of the magnet polarity. A damper (wire suspensions) 207 functioning as a current transmission unit and a tilt coil 217 are installed around the lens holder 202.
As shown in FIGS. 1, 2 and 3, each of the magnet assemblies 204 includes two -shaped magnets 204a and 204b that are symmetrically disposed each other and magnets 204c and 204d disposed on bottoms of the -shaped magnets 204a, the magnets 204c and 204d having a polarity opposite to the respective -shaped magnets 204a and 204b. 
The number of magnets and the polarities of the magnets may be varied.
The magnet assemblies 204 are fixed on an inner surface of the U-shaped yoke 203 formed of a ferromagnetic body.
First ends of the parallel wire suspensions 207 are fixed on a fixing portion 208 formed on the lens holder 202 and second ends of the parallel wire suspensions 207 are fixed on a main board (not shown) via a frame 209 spaced away from the lens holder 202.
The wire suspensions 207 functions to suspend the lens holder 102 and supply the current.
The operation of the prior optical pickup actuator will be described hereinafter.
Referring to FIGS. 1, 2 and 3, when the current is applied to the focusing coils 205, vertical magnetic flux is created, thereby performing the focusing drive of the lens holder.
At this point, the magnetic flux of the multi-polar magnet assemblies 204 facing the focusing coils 205 acts electro-magnetically to move the focusing coils 205 and the lens holder 202 in the focusing direction (in the vertical direction).
When the current is applied to the tracking coils 206, the multi-polar magnet assemblies 204 and the tracking coil 206 and the lens holder 202 are driven in the tracking direction (in the horizontal direction).
When the current is applied to the tilt coil 217 wound around the lens holder 202, as shown in FIG. 2, left and right sides of the lens holder 202 pivots in an opposite direction to each other by the opposite polarity between the magnets 204a and 204b opposing the tilt coil 217 according to Lorentz force.
The lens holder 202 should be designed to move in focusing and tracking directions perpendicular to each other without any vibration, rotation and twisting.
However, when the lens holder 202 is driven in the tracking direction, as shown in FIG. 3, since the weight center of the lens holder 202 does not accord with the tracking force center of the tracking coil 206, the lens holder 202 operates in a rolling mode at a high frequency range. As a result, the phase of the lens holder 202 is deformed.
That is, since the weight center in the vertical direction is a center of the lens holder while the weight center of the tracking direction is a center of the tracking, when the lens holder is driven in the tracking direction, the weight center in the tracking direction is varied due to the imbalance of the magnetic force.
In addition, the weight center may be offset upward from the center by the magnetic flux distribution formed by the multi-polar magnet assemblies. In order to solve this problem, a dummy mass (220 in FIG. 1) may be provided on a side portion of the top of the lens holder to heighten the weight center.
A tracking coil structure for solving the above-described problem will be described hereinafter with reference to FIGS. 4 through 6. The description of the same or like parts depicted in FIG. 1 will be omitted herein.
As shown in FIGS. 4A and 4B, the tracking coils 306 attached on the opposite surfaces of the lens holder 302 are formed in a trapezoid shape where the upper winding width is lesser than the lower winding width. That is, each of the tracking coils 306 has a triangular weight center.
In the operation, since the trapezoid tracking coils 306 are wound in a proper direction on the opposite surfaces of the lens holder 302 to generate attraction and repulsion by the electromagnetic force generated by the combination of the multi-polar magnet assemblies 304.
By the attraction and repulsion, the lens holder 302 is driven in the tracking direction (in the horizontal direction).
At this point, when the lens holder 302 is moved in the tracking direction by the tracking force TF in the rolling mode, the lens holder 302 horizontally pivots in the tracking direction by a reverse direction compensating torque TFc generated together with the tracking force TF by the tracking coil. That is, the trapezoid shape of the tracking coils 306 functions to lower the tracking center.
FIG. 5 shows a vector graph of the trapezoid tracking coils. As shown in the drawing, when the current is applied to the trapezoid tracking coils, the force F in the tracking direction is generated in a direction perpendicular to both inclined sides of the trapezoid tracking coils. That is, as the vector F is force (vector Fx+vector Fy, where the vector Fx is force in a direction of an X-axis and Fy is force in a direction a Y-axis) in the tracking direction.
Here, a torque causing vibration that causes the occurrence of the rolling by the vector Fx is Fx*bo where the bo is a difference between the tracking center TC and the weight center WC.
Generally, the Fx is greater than the Fy, it is possible to eliminate the occurrence of the rolling using the Fy.
At this point, the compensation torque (TFc) is 2*d1*Fy. Here, 2*d1 is a distance between the both sides of the tracking coil at a tracking force center line. The torque causing vibration, which causes the rolling is generated by the vector Fx and the reverse compensating torque is generated by the vector Fy. At this point, since the vector Fx is greater than Fy, the tracking sensitivity is not deteriorated and the rolling can be eliminated without increasing the mass of the lens holder.
Accordingly, the trapezoid tracking coils 306 are located on center portions of the opposite surfaces of the lens holder to drive the lens holder in the tracking direction by the force F (Fx+Fy) in the tracking direction and the compensation torque TFc(Fy). As a result, the tracking sensitivity is not deteriorated and the rolling can be eliminated without increasing the mass of the lens holder.
As a result, the rolling problem caused by the ascending of the tracking center of the tracking coils facing the 4-polar magnet assemblies from the weight center of the lens holder can be solved.
FIGS. 6A and 6B show driving states in the tracking direction and frequency properties of the rectangular tracking coil and the trapezoid tracking coil.
As shown in the drawings, by varying a shape of the tracking coil from the rectangular shape (206) to the trapezoid shape (306), the reverse torque TFc is generated by the offset between the tracking center TC and the weight center to eliminate residual torque, thereby preventing the rolling causing the vibration.
However, since the trapezoid coils have an asymmetrical structure, it cannot be wound around a bobbin. Therefore, the coils is first wound in the rectangular shape using a machine and then directly attached on the opposite surfaces of the lens holder.
To solve the above problem, as shown in FIG. 7a, the focusing coils are fixed on the lens holder 302 by bobbins 315 and the tracking coils 306 are hooked and fixed on upper and lower projections 318 and 319 that are integrally formed on the lens holder 302.
The upper projection 318 for the trapezoid tracking coil 306 is formed in an egg shape to minimize the contacting surface with the inner surface of the trapezoid tracking coil 306. In addition, the lower projection is formed in a circular shape to point-contact the inner surface of the trapezoid tracking coil 306. That is, the upper and lower projections 318 and 319 support the tracking coil 306 only in a vertical direction.
However, as shown in FIG. 7b, the tracking coil 306 is twisted in a direction as its attached location.
That is, since the lower projection 319 supports the tracking coil 306 through the point-contact, when the lower portion of the tracking coil 306 rotates, the upper portion of the tracking coil 306 also rotates, thereby twisting the tracking coil. As a result, there may be many problems in manufacturing the products.
In addition, when the tracking coil 306 is twisted from its normal position, abnormal electromagnetic force is generated between the tracking coil 306 and the magnet assembly, thereby making it difficult to obtain the normal tracking servo.